I don’t use a Mac, but the error your seeing is a common one apparently that affects many different NAS and other backup devices. Here is one such thread over on Apple’s support site (if you haven’t see it already) that goes back to 2012 with your issue.
https://discussions.apple.com/thread/4207296
There is also this Apple doc: https://support.apple.com/kb/PH11310?locale=en_US
This link may also be relevant: http://pondini.org/TM/C13.html
And this link:
https://community.wd.com/t/more-time-machine-errors/92548
And finally this link from 2011 may have a potential workaround:
http://www.garth.org/archives/2011,08,27,169,fix-time-machine-sparsebundle-nas-based-backup-errors.html
